1. Field of the Invention
The present invention concerns an aluminum alloy substrate for a lithographic printing plate of high mechanical strength and, more particularly, relates to an aluminum alloy substrate for a lithographic printing plate suitable to surface-roughening treatment and having excellent water retainablility and printing resistance.
2. Description of the Prior Art
Since plate materials made of aluminum or aluminum alloy, are light in weight, corrosion-resistant and easy to fabricate and also are well adapted for surface treatment, they have been used generally as substrates for lithographic printing plates.
As aluminum substrates for lithographic printing, plate materials made for example of AA1050 (at 99.5 wt % Al purity), AA1100 (at 99.0 wt % Al purity) and AA3003 (Al-0.05 to 0.2 wt % Cu-1.0 to 1.5 wt % Mn alloy) have often been used. These substrates are usually given a surface roughening treatment, for example, by means of a mechanical, chemical or electrochemical method for providing the surface with water retainability, then subjected to anodic oxidation and then coated with a light-sensitive composition, followed by drying to prepare a so-called PS plate. The PS plate is given the usual a plate-making treatment such as image exposure, development, rinse or lacquor coating and is then suitable for printing. The, the solubility to a liquid developer differs between the exposed area and the non-exposed area of the light-sensitive resin coated to the surface of the aluminum plate material, by which either one of the exposed area or the non-exposed area is removed, while the other of them remains to form an image on the aluminum plate as the substrate. The image area is ink receptive, while the hydrophilic surface of the aluminum substrate is uncovered in the non-image area from which the light-sensitive resin layer is dissolved and removed, to provide hydrophilicity. Then, the resultant printing plate is bent on both ends and clamped at the printing cylinder of a printing machine. Damping water is supplied to the surface of the printing plate and thereby maintains a film of damping water at the non-image area to repel oily ink therefrom and, on the other hand, the ink is attracted to an image area. Then printing is conducted by repeatedly transferring the ink deposited to the image area to a blanket and then transferring it to the surface of print paper.
For the aluminum plate material serve as the substrate for the lithographic printing have good mechanical strength for improving the press life and after a suitable roughening treatment be easily wettable with water with a sufficient water retention for preventing the deposition of the printing ink on the non-image area upon printing.
AA1000 series plate material generally used so far as the aluminum substrate for the lithographic printing plate has a satisfactory etching property and can easily provide a surface having good uniformity and excellent water retainability by the surface roughening treatment, for example, by means of an electrochemical method, but it suffers in the mechanical property of the plate material.
Recently, printing speed has increased with the advances in printing technique and stresses applied to a printing plate mechanically clamped at both ends to a printing cylinder of a printing machine are inevitably increased. Then, if the strength of the printing plate is insufficient, it has often led to the deformation or destruction in the clamped portions to bring about problems such as plate cracking, or the printing plate is disconnected to make printing impossible due to repeating stresses exerted on the bent portions of the printing plate. In view of the above, there is a need for an aluminum alloy substrate having higher mechanical strength for the lithographic printing plate.
With the aim as described above, attempts have been made to use AA3000 series of aluminum alloy having high mechanical strength (Al-Mn series alloy) as the material for the lithographic printing plate. For instance, Japanese Laid-Open Patent, Sho 60-230951 discloses a substrate for the printing plate using an aluminum alloy plate containing 0.05 to 1.0% by weight of Mn, not more than 0.20% by weight of Si and not more than 0.50% weight of Fe, and Japanese Laid-Open Patent, Hei 1-306288 discloses a substrate for the printing plate using an aluminum alloy plate containing 0.2 to 0.5% by weight of Si, 0.2 to 0.7% by weight of Fe, 0.3 to 1.5% by weight of Mn and not more than 0.05% by weight of Cu.
Although, the Al-Mn series aluminum alloy material is excellent as compared with AA1000 series alloy in mechanical strength, if it is used as the substrate for the lithographic printing plate, there has been a drawback in the homogenity or the hydrophilicity of the substrate surface obtianed by the surface roughening treatment, especially, by electrolytic etching or alkali etching, resulting in a non-etched portion, which makes it difficult to obtain a uniform hydrophilic surface and tends to cause ink stains in the non-image regions.